In order to elucidate the role of monocyte/macro-phage lineage (M/M) cells in HTLV-I infection, a human monocytic cell line, THP-1, was infected with HTLV-I by the co-culture with mitomycin-treated MT-2 cells. THP-1 can be infected by HTLV-I although the efficiency was not so high. A majority of population appeared to be lytically infected. A multi-step infection did not occur and HTLV-I genome was integrated in infected cells and only a small number of infected cells in a dormant state survived. These cells gained no growth advantage in comparison with the parental THP-1 cells and then cloning by the limiting dilution was necessary to obtain infected THP-1 cells. Infected clones originated from a single clone showed no viral messages or antigens. However, these clones expressed and secreted a fair amount of IL-6 when stimulated by lipopolysaccharide, compared with a parental cell, THP-1. These clones might be useful to investigate the dormant infection of HTLV-I in M/M cells the role of M/M cells in HTLV-I infection.